Geneva: UN-Arab League envoy Kofi Annan expects both the Syrian government and the opposition to fully implement a ceasefire agreement by April 12, his spokesman said on Thursday.

"What we expect on April 10 is that the Syrian government will have completed its withdrawal from populated centers ... and then we begin a 48-hour period during which there will be a complete cessation of all forms of violence by all parties," Ahmad Fawzi told reporters.

"So the clock starts ticking on the 10th on both sides to cease all forms of violence," he added. The Syrian regime has told Annan that it has begun withdrawing troops from some parts of the country, said Fawzi, adding that the three cities cited were Daraa and Abadani in the southwest and Idlib in the northwest.

"We are in the process of verification," he said. An advance team dispatched by Annan to pave the way for peacekeepers is also expected to arrive in Damascus on Thursday.

They will "begin discussing with the Syrian authorities the modalities of the eventual deployment of this UN supervision and monitoring mission" but will not be involved in verifying if troops have indeed begun withdrawing, said Fawzi.

The peacekeeping mission would be "ready to move in as soon as we have a Security Council resolution," he added.

Meanwhile, Annan will also travel on April 11 to Tehran, where he is to seek support from the Syrian ally for his peace plan.

Iran's supreme leader Ayatollah Khamanei has vowed to defend the Syrian regime and Tehran has provided President Bashar al-Assad's regime with political and material support in its brutal crackdown on a popular uprising that the UN estimates has left more than 9,000 people since mid-March last year.

Annan is due to address the UN General Assembly later on Thursday about the situation in Syria. The ex-UN chief, tapped as envoy in February, has previously briefed the UN Security Council twice on his mission in Syria.

Annan told the council on Monday that the Syrian government had accepted the deadline, and that he would push for an end of rebel operations within 48 hours after the government stops fighting. Annan had asked the council to support the deadline. He further said that Assad had agreed to immediately start pulling troops out of protest cities in order to meet the deadline.